The present invention relates to a separate lubricating device for an internal combustion engine, and in particular to a separate lubricating device for supplying a lubricating oil by means of an electronic control to an internal combustion engine, such as a relatively small two-stroke internal combustion engine which is suited for use in a portable working machine, for example, a chain saw, a bush cutter, or the like.
There have been previously known two different systems for supplying lubricating oil to a two-stroke internal combustion engine, namely: a mixture method lubrication system, which is widely employed, wherein lubricating oil is mixed in advance with fuel such that a mixture of fuel and lubricating oil is supplied to the engine intake; and a separate lubricating system, wherein a lubricating oil is supplied mechanically to the engine by a lubricating oil pump. Examples of separate lubricating systems are disclosed in, for example, Japanese Patent Unexamined Publication H1-113510; and Japanese Utility Model Unexamined Publication H2-13111.
The aforementioned mixture method lubrication system is designed to supply a lubricating oil at a predetermined mixing ratio, which is usually established to conform with the quantity of lubricating oil required for a high revolution speed of engine. It is difficult to adjust the quantity of lubricating oil to an optimum degree for low speed operation of the engine, thus causing problems such as the generation of smoke or offensive odor at low engine speeds or at idling of the engine.
On the other hand, with a conventional separate lubricating system, the lubricating oil pump is driven by motive power derived from a crankshaft, thereby to enable an optimum quantity of lubricating oil to be supplied from the discharge port of the pump to the internal combustion engine in conformity with the revolution speed of the engine. However, there is a problem that a fine control of supply of lubricating oil cannot be achieved by only controlling the revolution speed of the pump.
Furthermore, with the previously known apparatus for mechanically supplying lubricating oil, since the lubricating oil pump is driven by making use of the driving force of the engine, the pumping efficiency of the lubricating oil pump is caused to decrease as the revolution speed of the engine becomes higher, thus raising a problem that it becomes impossible to supply a sufficient quantity of lubricating oil which is necessary for a high revolution speed of engine.